LilaG writes: Drug tests spot banned substances based on their chemical structures, but a new breed of narcotics is designed to evade such tests. These synthetic marijuana drugs, found in "herbal incense," are mere chemical tweaks of each other, allowing them to escape detection each time researchers develop a new test for one of the compounds. Now chemists have developed a method that can screen for multiple designer drugs at once, without knowing their structures. The test may help law enforcement crack down on the substances.

The researchers used a technique called "mass defect filtering," which can detect related compounds all at once. That's because related compounds have almost equal numbers to the right of the decimal point in their molecular masses.

The researchers tested their technique on 32 herbal products with names like "Mr. Nice Guy" and "Hot Hawaiian." They found that every product contained one or more synthetic cannabinoid; all told, they identified nine different compounds in them — two illegal ones and seven that are not regulated.